Power electronic modules are semiconductor packages that are used in power electronic circuits. Power electronic modules are typically used in vehicular and industrial applications, such as in inverters and rectifiers. The semiconductor components included within the power electronic modules are typically insulated gate bipolar transistor (IGBT) semiconductor chips or metal-oxide-semiconductor field effect transistor (MOSFET) semiconductor chips. The IGBT and MOSFET semiconductor chips have varying voltage and current ratings. Some power electronic modules also include additional semiconductor diodes (i.e., free-wheeling diodes) in the semiconductor package for overvoltage protection.
In general, two different power electronic module designs are used. One design is for higher power applications and the other design is for lower power applications. For higher power applications, a power electronic module typically includes several semiconductor chips integrated on a single substrate. The substrate typically includes an insulating ceramic substrate, such as Al2O3, AlN, Si3N4, or other suitable material, to insulate the power electronic module. At least the top side of the ceramic substrate is metallized with either pure or plated Cu, Al, or other suitable material to provide electrical and mechanical contacts for the semiconductor chips. The metal layer is typically bonded to the ceramic substrate using a direct copper bonding (DCB) process, a direct aluminum bonding (DAB) process, or an active metal brazing (AMB) process.
Typically, soft soldering with Sn—Pb, Sn—Ag, Sn—Ag—Cu, or another suitable solder alloy is used for joining a semiconductor chip to a metallized ceramic substrate. Typically, several substrates are combined onto a metal base plate. In this case, the backside of the ceramic substrate is also metallized with either pure or plated Cu, Al, or other suitable material for joining the substrates to the metal base plate. To join the substrates to the metal base plate, soft soldering with Sn—Pb, Sn—Ag, Sn—Ag—Cu, or another suitable solder alloy is typically used.
For vehicular applications, such as hybrid electric vehicles, the coolant of the combustion engine may be used for cooling the power semiconductor modules. Junction temperatures up to 200° C. may be exhibited within the power semiconductor chips. The solder layer between the substrate and the metal base plate experience the temperature of the coolant plus approximately 10° C., which results from the thermal impedance from the substrate to the metal base plate and coolant. The temperature at the substrate is typically around 110° C., but may reach a maximum temperature of up to around 140° C. Therefore, compared to a typical industrial application, the solder layer may experience a wider range of temperature and a temperature swing of approximately 30° C. to 60° C. more than for the industrial application. The additional 30° C. to 60° C. in temperature swing roughly doubles the temperature swing compared to a typical industrial application.
Due to the wide temperature swing during thermal cycling, the lifetime of the power electronic module may be reduced. Cracks may form inside the solder layer after repeated thermal cycles. The cracks can easily spread over the entire solder layer and lead to the failure of the power electronic module. With the increasing desire to use power electronics in harsh environments (e.g., automotive applications) and the ongoing integration of semiconductor chips, the temperature swing experienced by the power electronic modules will continue to increase. Therefore, there is a growing demand for power electronic modules capable of withstanding thermal cycling having a temperature swing greater than or equal to approximately 100° C.
For these and other reasons, there is a need for the present invention.